mirror of
https://github.com/ethereum/solidity
synced 2023-10-03 13:03:40 +00:00
399 lines
14 KiB
ReStructuredText
399 lines
14 KiB
ReStructuredText
.. index:: ! functions
|
|
|
|
.. _functions:
|
|
|
|
*********
|
|
Functions
|
|
*********
|
|
|
|
.. _function-parameters-return-variables:
|
|
|
|
Function Parameters and Return Variables
|
|
========================================
|
|
|
|
As in JavaScript, functions may take parameters as input. Unlike in JavaScript
|
|
and C, functions may also return an arbitrary number of values as output.
|
|
|
|
Function Parameters
|
|
-------------------
|
|
|
|
Function parameters are declared the same way as variables, and the name of
|
|
unused parameters can be omitted.
|
|
|
|
For example, if you want your contract to accept one kind of external call
|
|
with two integers, you would use something like::
|
|
|
|
pragma solidity >=0.4.16 <0.7.0;
|
|
|
|
contract Simple {
|
|
uint sum;
|
|
function taker(uint _a, uint _b) public {
|
|
sum = _a + _b;
|
|
}
|
|
}
|
|
|
|
Function parameters can be used as any other local variable and they can also be assigned to.
|
|
|
|
.. note::
|
|
|
|
An :ref:`external function<external-function-calls>` cannot accept a
|
|
multi-dimensional array as an input
|
|
parameter. This functionality is possible if you enable the new
|
|
experimental ``ABIEncoderV2`` feature by adding ``pragma experimental ABIEncoderV2;`` to your source file.
|
|
|
|
An :ref:`internal function<external-function-calls>` can accept a
|
|
multi-dimensional array without enabling the feature.
|
|
|
|
.. index:: return array, return string, array, string, array of strings, dynamic array, variably sized array, return struct, struct
|
|
|
|
Return Variables
|
|
----------------
|
|
|
|
Function return variables are declared with the same syntax after the
|
|
``returns`` keyword.
|
|
|
|
For example, suppose you want to return two results: the sum and the product of
|
|
two integers passed as function parameters, then you use something like::
|
|
|
|
pragma solidity >=0.4.16 <0.7.0;
|
|
|
|
contract Simple {
|
|
function arithmetic(uint _a, uint _b)
|
|
public
|
|
pure
|
|
returns (uint o_sum, uint o_product)
|
|
{
|
|
o_sum = _a + _b;
|
|
o_product = _a * _b;
|
|
}
|
|
}
|
|
|
|
The names of return variables can be omitted.
|
|
Return variables can be used as any other local variable and they
|
|
are initialized with their :ref:`default value <default-value>` and have that value unless explicitly set.
|
|
|
|
You can either explicitly assign to return variables and
|
|
then leave the function using ``return;``,
|
|
or you can provide return values
|
|
(either a single or :ref:`multiple ones<multi-return>`) directly with the ``return``
|
|
statement::
|
|
|
|
pragma solidity >=0.4.16 <0.7.0;
|
|
|
|
contract Simple {
|
|
function arithmetic(uint _a, uint _b)
|
|
public
|
|
pure
|
|
returns (uint o_sum, uint o_product)
|
|
{
|
|
return (_a + _b, _a * _b);
|
|
}
|
|
}
|
|
|
|
This form is equivalent to first assigning values to the
|
|
return variables and then using ``return;`` to leave the function.
|
|
|
|
.. note::
|
|
You cannot return some types from non-internal functions, notably
|
|
multi-dimensional dynamic arrays and structs. If you enable the
|
|
new experimental ``ABIEncoderV2`` feature by adding ``pragma experimental
|
|
ABIEncoderV2;`` to your source file then more types are available, but
|
|
``mapping`` types are still limited to inside a single contract and you
|
|
cannot transfer them.
|
|
|
|
.. _multi-return:
|
|
|
|
Returning Multiple Values
|
|
-------------------------
|
|
|
|
When a function has multiple return types, the statement ``return (v0, v1, ..., vn)`` can be used to return multiple values.
|
|
The number of components must be the same as the number of return types.
|
|
|
|
.. index:: ! view function, function;view
|
|
|
|
.. _view-functions:
|
|
|
|
View Functions
|
|
==============
|
|
|
|
Functions can be declared ``view`` in which case they promise not to modify the state.
|
|
|
|
.. note::
|
|
If the compiler's EVM target is Byzantium or newer (default) the opcode
|
|
``STATICCALL`` is used for ``view`` functions which enforces the state
|
|
to stay unmodified as part of the EVM execution. For library ``view`` functions
|
|
``DELEGATECALL`` is used, because there is no combined ``DELEGATECALL`` and ``STATICCALL``.
|
|
This means library ``view`` functions do not have run-time checks that prevent state
|
|
modifications. This should not impact security negatively because library code is
|
|
usually known at compile-time and the static checker performs compile-time checks.
|
|
|
|
The following statements are considered modifying the state:
|
|
|
|
#. Writing to state variables.
|
|
#. :ref:`Emitting events <events>`.
|
|
#. :ref:`Creating other contracts <creating-contracts>`.
|
|
#. Using ``selfdestruct``.
|
|
#. Sending Ether via calls.
|
|
#. Calling any function not marked ``view`` or ``pure``.
|
|
#. Using low-level calls.
|
|
#. Using inline assembly that contains certain opcodes.
|
|
|
|
::
|
|
|
|
pragma solidity >=0.5.0 <0.7.0;
|
|
|
|
contract C {
|
|
function f(uint a, uint b) public view returns (uint) {
|
|
return a * (b + 42) + now;
|
|
}
|
|
}
|
|
|
|
.. note::
|
|
``constant`` on functions used to be an alias to ``view``, but this was dropped in version 0.5.0.
|
|
|
|
.. note::
|
|
Getter methods are automatically marked ``view``.
|
|
|
|
.. note::
|
|
Prior to version 0.5.0, the compiler did not use the ``STATICCALL`` opcode
|
|
for ``view`` functions.
|
|
This enabled state modifications in ``view`` functions through the use of
|
|
invalid explicit type conversions.
|
|
By using ``STATICCALL`` for ``view`` functions, modifications to the
|
|
state are prevented on the level of the EVM.
|
|
|
|
.. index:: ! pure function, function;pure
|
|
|
|
.. _pure-functions:
|
|
|
|
Pure Functions
|
|
==============
|
|
|
|
Functions can be declared ``pure`` in which case they promise not to read from or modify the state.
|
|
|
|
.. note::
|
|
If the compiler's EVM target is Byzantium or newer (default) the opcode ``STATICCALL`` is used,
|
|
which does not guarantee that the state is not read, but at least that it is not modified.
|
|
|
|
In addition to the list of state modifying statements explained above, the following are considered reading from the state:
|
|
|
|
#. Reading from state variables.
|
|
#. Accessing ``address(this).balance`` or ``<address>.balance``.
|
|
#. Accessing any of the members of ``block``, ``tx``, ``msg`` (with the exception of ``msg.sig`` and ``msg.data``).
|
|
#. Calling any function not marked ``pure``.
|
|
#. Using inline assembly that contains certain opcodes.
|
|
|
|
::
|
|
|
|
pragma solidity >=0.5.0 <0.7.0;
|
|
|
|
contract C {
|
|
function f(uint a, uint b) public pure returns (uint) {
|
|
return a * (b + 42);
|
|
}
|
|
}
|
|
|
|
Pure functions are able to use the `revert()` and `require()` functions to revert
|
|
potential state changes when an :ref:`error occurs <assert-and-require>`.
|
|
|
|
Reverting a state change is not considered a "state modification", as only changes to the
|
|
state made previously in code that did not have the ``view`` or ``pure`` restriction
|
|
are reverted and that code has the option to catch the ``revert`` and not pass it on.
|
|
|
|
This behaviour is also in line with the ``STATICCALL`` opcode.
|
|
|
|
.. warning::
|
|
It is not possible to prevent functions from reading the state at the level
|
|
of the EVM, it is only possible to prevent them from writing to the state
|
|
(i.e. only ``view`` can be enforced at the EVM level, ``pure`` can not).
|
|
|
|
.. note::
|
|
Prior to version 0.5.0, the compiler did not use the ``STATICCALL`` opcode
|
|
for ``pure`` functions.
|
|
This enabled state modifications in ``pure`` functions through the use of
|
|
invalid explicit type conversions.
|
|
By using ``STATICCALL`` for ``pure`` functions, modifications to the
|
|
state are prevented on the level of the EVM.
|
|
|
|
.. note::
|
|
Prior to version 0.4.17 the compiler did not enforce that ``pure`` is not reading the state.
|
|
It is a compile-time type check, which can be circumvented doing invalid explicit conversions
|
|
between contract types, because the compiler can verify that the type of the contract does
|
|
not do state-changing operations, but it cannot check that the contract that will be called
|
|
at runtime is actually of that type.
|
|
|
|
.. index:: ! fallback function, function;fallback
|
|
|
|
.. _fallback-function:
|
|
|
|
Fallback Function
|
|
=================
|
|
|
|
A contract can have exactly one unnamed function. This function cannot have
|
|
arguments, cannot return anything and has to have ``external`` visibility.
|
|
It is executed on a call to the contract if none of the other
|
|
functions match the given function identifier (or if no data was supplied at
|
|
all).
|
|
|
|
Furthermore, this function is executed whenever the contract receives plain
|
|
Ether (without data). To receive Ether and add it to the total balance of the contract, the fallback function
|
|
must be marked ``payable``. If no such function exists, the contract cannot receive
|
|
Ether through regular transactions and throws an exception.
|
|
|
|
In the worst case, the fallback function can only rely on 2300 gas being
|
|
available (for example when `send` or `transfer` is used), leaving little
|
|
room to perform other operations except basic logging. The following operations
|
|
will consume more gas than the 2300 gas stipend:
|
|
|
|
- Writing to storage
|
|
- Creating a contract
|
|
- Calling an external function which consumes a large amount of gas
|
|
- Sending Ether
|
|
|
|
Like any function, the fallback function can execute complex operations as long as there is enough gas passed on to it.
|
|
|
|
.. note::
|
|
Even though the fallback function cannot have arguments, one can still use ``msg.data`` to retrieve
|
|
any payload supplied with the call.
|
|
|
|
.. warning::
|
|
The fallback function is also executed if the caller meant to call
|
|
a function that is not available. If you want to implement the fallback
|
|
function only to receive ether, you should add a check
|
|
like ``require(msg.data.length == 0)`` to prevent invalid calls.
|
|
|
|
.. warning::
|
|
Contracts that receive Ether directly (without a function call, i.e. using ``send`` or ``transfer``)
|
|
but do not define a fallback function
|
|
throw an exception, sending back the Ether (this was different
|
|
before Solidity v0.4.0). So if you want your contract to receive Ether,
|
|
you have to implement a payable fallback function.
|
|
|
|
.. warning::
|
|
A contract without a payable fallback function can receive Ether as a recipient of a `coinbase transaction` (aka `miner block reward`)
|
|
or as a destination of a ``selfdestruct``.
|
|
|
|
A contract cannot react to such Ether transfers and thus also cannot reject them. This is a design choice of the EVM and Solidity cannot work around it.
|
|
|
|
It also means that ``address(this).balance`` can be higher than the sum of some manual accounting implemented in a contract (i.e. having a counter updated in the fallback function).
|
|
|
|
::
|
|
|
|
pragma solidity >=0.5.0 <0.7.0;
|
|
|
|
contract Test {
|
|
// This function is called for all messages sent to
|
|
// this contract (there is no other function).
|
|
// Sending Ether to this contract will cause an exception,
|
|
// because the fallback function does not have the `payable`
|
|
// modifier.
|
|
function() external { x = 1; }
|
|
uint x;
|
|
}
|
|
|
|
|
|
// This contract keeps all Ether sent to it with no way
|
|
// to get it back.
|
|
contract Sink {
|
|
function() external payable { }
|
|
}
|
|
|
|
contract Caller {
|
|
function callTest(Test test) public returns (bool) {
|
|
(bool success,) = address(test).call(abi.encodeWithSignature("nonExistingFunction()"));
|
|
require(success);
|
|
// results in test.x becoming == 1.
|
|
|
|
// address(test) will not allow to call ``send`` directly, since ``test`` has no payable
|
|
// fallback function. It has to be converted to the ``address payable`` type via an
|
|
// intermediate conversion to ``uint160`` to even allow calling ``send`` on it.
|
|
address payable testPayable = address(uint160(address(test)));
|
|
|
|
// If someone sends ether to that contract,
|
|
// the transfer will fail, i.e. this returns false here.
|
|
return testPayable.send(2 ether);
|
|
}
|
|
}
|
|
|
|
.. index:: ! overload
|
|
|
|
.. _overload-function:
|
|
|
|
Function Overloading
|
|
====================
|
|
|
|
A contract can have multiple functions of the same name but with different parameter
|
|
types.
|
|
This process is called "overloading" and also applies to inherited functions.
|
|
The following example shows overloading of the function
|
|
``f`` in the scope of contract ``A``.
|
|
|
|
::
|
|
|
|
pragma solidity >=0.4.16 <0.7.0;
|
|
|
|
contract A {
|
|
function f(uint _in) public pure returns (uint out) {
|
|
out = _in;
|
|
}
|
|
|
|
function f(uint _in, bool _really) public pure returns (uint out) {
|
|
if (_really)
|
|
out = _in;
|
|
}
|
|
}
|
|
|
|
Overloaded functions are also present in the external interface. It is an error if two
|
|
externally visible functions differ by their Solidity types but not by their external types.
|
|
|
|
::
|
|
|
|
pragma solidity >=0.4.16 <0.7.0;
|
|
|
|
// This will not compile
|
|
contract A {
|
|
function f(B _in) public pure returns (B out) {
|
|
out = _in;
|
|
}
|
|
|
|
function f(address _in) public pure returns (address out) {
|
|
out = _in;
|
|
}
|
|
}
|
|
|
|
contract B {
|
|
}
|
|
|
|
|
|
Both ``f`` function overloads above end up accepting the address type for the ABI although
|
|
they are considered different inside Solidity.
|
|
|
|
Overload resolution and Argument matching
|
|
-----------------------------------------
|
|
|
|
Overloaded functions are selected by matching the function declarations in the current scope
|
|
to the arguments supplied in the function call. Functions are selected as overload candidates
|
|
if all arguments can be implicitly converted to the expected types. If there is not exactly one
|
|
candidate, resolution fails.
|
|
|
|
.. note::
|
|
Return parameters are not taken into account for overload resolution.
|
|
|
|
::
|
|
|
|
pragma solidity >=0.4.16 <0.7.0;
|
|
|
|
contract A {
|
|
function f(uint8 _in) public pure returns (uint8 out) {
|
|
out = _in;
|
|
}
|
|
|
|
function f(uint256 _in) public pure returns (uint256 out) {
|
|
out = _in;
|
|
}
|
|
}
|
|
|
|
Calling ``f(50)`` would create a type error since ``50`` can be implicitly converted both to ``uint8``
|
|
and ``uint256`` types. On another hand ``f(256)`` would resolve to ``f(uint256)`` overload as ``256`` cannot be implicitly
|
|
converted to ``uint8``.
|